1. Field of the Invention
The present invention relates to an engine idling rotational speed control device, and more particularly to an engine rotational speed control device for controlling an idling rotational speed of an engine.
2. Description of the Prior Art
In an engine such as an automobile engine, an engine rotational speed control device is usually used in order to maintain smooth rotation of the engine even when a throttle valve is in a full-close position by removal of a foot of a driver from an accelerator pedal in a non-load condition. One type of the engine rotational speed control device comprises intake air flow rate control means for controlling an intake air quantity in the idling position of the engine or the full-close position of the throttle valve, a stepping motor for driving the intake air flow rate control means and an electronic control circuit for controlling the stepping motor in accordance with the operating condition of the engine.
When such a device is used in an engine having an electronically controlled fuel injection device including a throttle body in which the throttle valve for controlling the intake air quantity is arranged and a fuel injection device for injecting fuel to air suctioned into an intake manifold through the throttle body, a bypass passage for bypassing the throttle valve of the throttle body is provided to control the opening area of the bypass passage by an intake air flow rate control valve driven by the stepping motor in the idling condition or the full-close position of the throttle valve in order to control the intake air quantity in the idling condition. With the idling rotational speed control device provided with the air flow rate valve driven by the step motor as described above, in the electronic control circuit, the increase or decrease in pulse number for driving the step motor from the reference position is calculated, whereby the opening degree of the air flow rate control valve is brought into register with the position of the step motor stored in the electronic control circuit, so that the opening degree of the air flow rate control valve can be detected.
As shown in FIG. 1, in the prior art device, when an ignition switch is turned off from its on position at a time t.sub.1, the stepping motor is driven to fully open the control valve at a time t.sub.2 in order to prevent freeze in a low temperature environment. As the engine is in a full operation at a time t.sub.3, the flow rate control valve is abruptly closed by the stepping motor (see a time period between t.sub.3 and t.sub.4) to attain an idling rotational speed set in accordance with an engine coolant temperature. After a time t.sub.4, the step position of the stepping motor is corrected such that a target idling rotational speed for the engine coolant temperature is attained and the step position of the stepping motor is controlled to maintain the target idling rotational speed.
In the above-described device, judgements is made as to whether the engine idling rotational speed should be controlled or not on the basis of a detection of the throttle valve substantially fully closed. However, in general, the above-described control is effected when an accelerator pedal is not only released but also slightly depressed, i.e., the throttle valve is slightly opened. More specifically, under a condition where the throttle valve is slightly opened in a idling operation, the step position of the stepping motor is decremented to reduce the intake air quantity from the intake air flow rate control means so as to maintain the target idling engine rotational speed. Accordingly, if the throttle valve is fully closed by releasing the accelerator pedal after the throttle valve is slightly opened, the engine may stall due to lack of an air flow rate introduced into the engine via the bypass passage.